Ancalagon, my best guess from what I've read at various sources is that magic items needs some kind of maintenance (potions need to be re-brewed every few days or their enchantment runs out, Dresden's rings need to have him keep moving, Little Chicago requires updating when new buildings are built, Evil Jim's Stick O' Doom requires regular infusions of puppy blood, etc.) and this is abstracted out by having a limit on the total power that all of a wizard of a given power's ("power" determined here by the number of magical Stunts the wizard has taken, I think) magical gadgets can have (to keep PC wizards from making 100 rings like Dresden's and putting them on all of their toes and coating their fingers with them, etc.).
In the character sheet Dresden seems to have a maintenance limit of 5 magic gizmos and he's spent three on his rod, staff and shield bracelet and has two slots open for random potions. If he tried to make 10 magic gizmos, then he's just not enough of a wizard to keep them up and running, just like I'm not enough of a car mechanic to keep 10 constantly used cars up and running.
Now what happens if he loses some? Well then he's lost one, but he hasn't lost any XP, since he's just freed up a slot that he can fill up again by making something new during the downtime between adventures. So this way the wizard doesn't get screwed over permanently, he just loses that item for a while and he's be back to normal at the start of the next adventure (just like what happens to Dresden when he loses his rod/staff).
The second part of your question is what about people getting a big power boost from stealing magic shit. In the Dresden Books Harry never gives any of his allies any magic items except for potions so we can assume that either:
A. Most magic items don't work (or at least don't work for very long) when not used by a magical practitioner (or maybe even when not used by the specific person they were made for).
or.
B. Harry is a real prick since he never gave Murphy even the smallest form of magical protection item, not even a bullet-reflecting jacket or something.
The RPG makers are going with assumption A
According to Fred, there's two kinds of magic items "focus" items (which help you cast spells better, like Harry's staff) and "enchantment" items (which store a spell and can be only used once and seem to wear off after a few days if they're not used and not recharged, like Harry's potions and possibly his rings).
Often it seems that these enchantment items can be used by anyone but they have a limited duration and a one-time use so they wouldn't be overpowering in play and they often seem to require some magic in order to trigger them (I don't think that Murphey could use one of Harry's rings for example).
Now this goes for all of someone's standard magical gear that can be (pretty) easily replaced. Slightly different rules probably apply for unique/irreplaceable/character-defining stuff like Michael's sword.
I THINK all of that's right, but I'm probably missing some details.
I can think of two upshots of all of this:
A. A potion-brewer would be an interesting character. He'd have to make the potions ahead of time but not too much ahead of time (since they only last a few days) and then dole them out.
B. All of this gives me great ideas for a Norse-style rune-carver. Most runes were made to give someone a bonus for just one specific even (i.e. "help Bob during his battle against Jim tomorrow by making him stronger") not general boosters (i.e. "make Bob stronger"). Most runes would then follow potion rules and a clever rune-carver would be well-served to make runes with a short duration (since they have a limit to the number they can have active at once) and with lots of specific conditions (to manipulate people into doing what the runecarver wants). A runecarver would only create a permanent effect when they REALLY care about something since that'd use up one of their magic gizo slots (for example with Egil Skallagrimson carves a nith rune against the Norwegian king since he hates the king's guts). All of this would provides mechanical incentives for people to act like Norse rune carvers did, which is really cool. I could see that putting together some Norse magic houserules for Dresden would be dead easy.
